Pneumatic hammer



0d. 29, 1940. DUNCAN I 2,219,739

' PNEUMATIC HAMMER Filed Aug. 30, 1937 2 Sheets-Sheet l 7 0 9 6 a; b f 2 a;

65:41AM, cz M Patented Oct. 29,1940

UNITED STATES PNEUMATIO. HAMMER Forrest M. Duncamchicago, Ill.

Application August 30,

10 Claims.

My invention relates generally to automatic mechanisms for driving tacks, nails, brads or the like, and has particular relation to portable devices of this type. r

In the upholstering and similar arts, there has long been a great need for automatic tack or nail driving equipment provided with means whereby a plurality of tacks or nails might be driven in succession, i. e. for an automatic tack or nail driving mechanism of a repeating type,- and the principal object of the present invention is toprovide such a device.

Other objects of myinvention are to provide a simple,- automatic, easily operated tack or nail driving mechanism of the repeating type which shall embody a magazine .of novel construction which is easily and-quickly removable from the mechanism when empty to permit the substitur. tion therefor of a new magazine; andto provide automatic, repeating type tack or nail driving mechanism which shall be'capable of imparting as many repeated blows to any particular tack or nail being driven as the operator may desire without in any way interfering with the automatic and successive feeding of subsequent tacks during the normal operation of the device, Additional objects and advantages of my inventionwill be made apparent by reference to the accompanying drawings and the following description of one preferred embodiment thereof.

In the draWings- Figure l is a perspective view of an automatic tack drivingmechanism of a repeating type embodying the features of the present invention;

Figure 2 is an axial, enlarged sectional view of the mechanism illustrated generally in Figure l;

Figures 3 and 4 are perspective views illustrating the structural features of certain elements of the grip frame constituting a part of the mechanism illustrated in Figures 1 and 2;

FigureS is a sectional view on the line 5-5 of Figure 1 illustrating certain of the features of the magazine and the driving hammer forming a part of the mechanism of my invention;

' Figures 6 and 7 are sectional views on the lines 6-6 and 1-1, respectively, of Figure 2;

Figure 8 is a fragmentary view illustrating the vmagnetization of the magazine used in conjunction with the mechanism of my invention;

Figure 9 is a fragmentary perspective view of the injector forming a part of the mechanism of my invention;

Figure 10 is a fragmentary view, partly in section, w g the relative p s t on 9 the s veral 1937, Serial No. 161,537 (01.1-46.1)

elements constituting the indexing mechanism for the magazine in normal or rest position;

Figure 11 is a fragmentary view similar to Figure 6 illustrating the relative position of the several elements of the-indexing mechanism 'at th'e conclusion of an indexing operation;

Figure 12 is a perspective view of the pawl operating slide constituting a part of the indexing mechanism and the magazine;

Figure 13 is a perspective view of the trigger element;

, Figure 14 isa fragmentary view similar to Figures 6 and '7, illustrating the position of the various elements of the mechanism, just prior to the operation of the driving hammer; and

Figure 15 is a fragmentary view illustrating the position of the various movable elements of the mechanism as a whole, following the operation of the driving hammer. o

The mechanism illustrated in the drawings is particularly adapted for driving carpet tacks or upholstery-tacks; It will be understood, however, that a similarly arranged structure can be used for driving nails, brads, or the like. Hence, when the Word tack is used herein, it should be understood to include broadly tacks, nails, brads and the like.

Referring to the drawings, the device illustrated therein includes a hammer or driving means which is pneumatically operated, a magazine for holding a supply of tacks, and operating mechanism for effecting and controlling the operation of the various elements of the device: The operating mechanism includes mechanism; for indexing the magazine so as to permiteach of the tacks contained therein to be movedsuc cessively into operative relation with the tack driving means, and valve means actuable to control the operation of the driving hammer in such manner that single or multiple blows may be struck during the. driving of any particular tack.

More specifically, the device includes a metallic frame 2|. which for convenience has the general form of a pistol grip. The frame 2| comprises a hollow inner section 23 sandwichedbetween, two outer plate members 24 and 25, the upper ends of which are provided with arcuate shaped housing portions 21 (Figures 1 and 3) which in p the assembled device define the walls of a magazine chamber 28. The indexing and valve mechanism for controlling the operation of the hammer are contained within the hollow, grip frame 2| and are adapted to be operated by a slidable trigger member 29 as will be described in more detail in a subsequent paragraph. 1

The hammer or driving mechanism constitutes a unitary structure pivotally connected to-the grip frame 2| by a suitable pin 30 Figure 5 and normally held in the position illustrated in 5 Figures 2 and 15 by a releasable latch 30a. The

' hammer mechanism includes a barrel 3| integrally attached to a piston cylinder 33. A circular plate member 34 (Figures 1 and '1) disposed beneath the barrel 3| serves to close the top of the operative position.

A slide valve 35 (see Figure 2) of conventional design is rigidly attached to the bottom of the piston cylinder 33 by suitable means such as is biased to the normal position thereof by means of a compression spring 39, one end of which abuts against the head 40 of the valve operating member 42, A swivel type air hose connection 43 connecting the driving mechanism to a source of pressure air.

A suitably proportioned cylindrical piston 45-is arranged for movement within the piston cylinder 33. If desired, this piston 45 may be fitted with leather sealing washers 46 or the like for engaging the walls of the piston chamber; The hammer proper'(41) constitutes a cylindrical rod aflix'ed at one end to the piston 45 and adapted to move within the barrel 3|.

and is magnetized in order to aid in holding the tacks'or nails in proper position during operation of the mechanism. The' rear end of the piston chamber 33 is closed by a suitable plug 49, and if desired a resilient bumper indicated at 59 maybe provided for minimizing shocks during the return of the hammer and pistonatthe conclusion .of

each stroke.

40 A complete operation .of the apparatus of the present invention is efiected by moving the trigger or operating member 29 from the normal or outer extreme position of that member illustrated in Figures 2 and 10 to an inner extreme position illustrated in Figure 15. There are two important l intermediate positions of the trigger or operating member 29. Movement of the trigger 29 from the outer extreme position (Figures 2 and 10) t-o'the first intermediate position which is illustrated in Figure 11 operates the indexing mechanism;

movement of the trigger from the first intermediate position (Figure 11) to the second intermedi- 'ate position which is-illustrated in Figure 14 effects the operation of the tack feeding mecha nism; and movement of the trigger from the-second intermediate position (Figure 14) to the extreme inner position (Figure 15) effects the operation of the tack driving mechanism.

Suitable inlet-outlet passageways 5| and 52, connecting passageways 5|a, 5lb, 52a and 52b,

and a vent passageway 53- (Figures 2 and 15) cooperate with the movable element 31 of the slide valve 35 to assure proper movement of the piston 45 and hammer 41 in response to movement of the trigger 29 and the valve operating member 42.

When all of the elements of the mechanism are in the'normal position illustrated in Figure 2, the

piston 45 and hammer 41 are heldin a retracted position bypressure air which is admitted to the inner (left hand) end of the piston chamber 45a,

the air passing from the supply line through the valve chamber 35a, through the short 'passage way 5|a into the passageway 5|, and thence through the short passageway Slate the inner end of the piston chamber 45a. The outer (right magazine chamber 28 when the mechanism is inscrews. The slidable element 31 of the valve 35- The outer end section 48. of the hammer 41 is formed of magnetic materialhand) end ofthe piston chamber 45a is vented to the atmosphere through the short passageway portion of the slide element 31, and the vent,

passageway 53.

Movement of the trigger member from the second intermediate position of Figure 14 to the extreme inner position .of Figure 15 effects movementof the slide valve from theposition of'Figure 2 to the position of Figure 15. In the Figu-.: 15 position, pressure air passes from the supply line through the valve chamber 35a; through the short passageway 52b, the-passageway 52 and the short passageway52a into the outer (right hand) end of the piston chamber 45. Simultaneously, the'inner (left hand) end of the piston chamber 45a is vented to the atmosphere'thro'ugh the short I passageway 5|b, the passageway 5|, the short is located on the lower side of slide valve 35-for mer 41 are moved forward by the pressure air at high velocity, carrying the tack previously introduced into the barrel on the end of the hammer. If desired, glands or other sealsmay' be provided about the hammer 41 to minimize air leakage.

, .To restore the hammer 41 to the retractedor driving position, the trigger member is moved fromthe extreme inner position of Figure 15 to the second intermediate position of Figure 14. The effect of this movement is twofold: Pressure air is cut off from the inner end of the piston chamber 451! and the inner end of the piston chamber issimultaneously vented to the atmosphere through the short passageway 52a, the passageway 52, the short passageway 52b, the chamber formed by the recess 55 in the inner portion of the slide element 31,and the vent passageway 53.

At the same time, pressure air is conducted from the supply line through the valve chamber 35a to the inner end of the piston chamber 45a through the short passageway 5|a, the passageway 5 I, and

the short passageway 5). The net effect of this operation is to cause the retraction of the piston 4,5 and the hammer 41 connected thereto, and if desired, repeated blows may'be struck by the hammer 41' by moving the trigger member 29 be tween the second intermediate position and the inner extreme position. Each movement of the trigger from the second intermediate position to the inner extreme position will cause the hammer 41 to move forward and produce adriving blow, and each movement of the trigger member from the inner extreme position to the second intermediate position will retract the hammer and render the mechanism in condition to deliver another driving blow if desired. The only way that a tack can be fed into the mechanism is to fully release the trigger to the extreme outer position and then move the trigger from the extreme outer position to the second intermediate position.

The tack or nail supply is carried in acircular,

plate-like magazine 59 provided with a plurality of radial slots 60, as illustrated particularly in Figures 5'andl2. The magazine 59 is preferably made of mild steel or other magnetic mate- .rial and is magnetized so that alternate spokes are of difierent polarity as indicated in Figure 8.

This magnetizing of the spokes serves to hold the tacks in place during the handling of .the magazine-and during the operation of theinjecting and hammer mechanism. It is desirable to provide a' peripheral groove 6| at the edgeof the magazine 59 in order that the tacks held in the magazine will not become displaced. The central portion of the under side of the magazine 58 is provided with a plurality of radial teeth 88 for cooperating with the indexing mechamsm, and with a centrally disposed hole for engaging a pin 85 integral with the inner section 25 of the grip frame. During the operation of the device, the magazine 58 rotates on this pin 85 in response to the operation of the indexing mechanism. When all of the tacks carried by a magazine have been driven, the empty magazine can be easily removed and a filled magazine substituted therefor merely by releasing the ,latch 38a and moving the unitary driving mechanism to the position illustrated by dot and dash linesin ,Figure 15, this movement serving to break the entire mechanism similarly to a repeating pistol.

The indexing mechanism is illustrated particularly in Figures 7,10, 11, 12 and 13, and includes a stationary pawl 81 for preventing reverse movement of the magazine during the indexing operation. The stationary pawl 81 is mounted upon the grip frame directly beneath the magazine chamber 28, and the outer end thereof projects through an opening 68 (Figure 3) in the frame section 25. The pawl 81 isvspring biased into engagement with the radial teeth 83' on theunderside of the magazine 58. The index'ing mechanism also includes an operating pawl 88 which is mounted on a slide member II as is shown in Figures 7, l0, and 12, the slide member 1| being supported for slidable movement upon the rear section 25 of the grip frame by means of a pin I3 which engages a slot 14 formed in the member 1| and a small removable guide plate 15. The slide member 1| which carries the operating pawl 68 is biased to the normal position by means of. a fiat spring 11. Movement of the slide 40 member H and the operating pawl 88 in response to movement of the trigger member 28 is effected by a triangularly shaped crank 18 pivotally supported upon the guide plate 15. The crank member 78 is provided with a pair of projecting roller members 8| one of which engages a vertical slot with the outwardly projecting rollers 8|. movement of the crank'member 18 will be transthe triangularly' shaped crank 18 will be rotated clockwise (Figure 10) about 30 due to the engagement oi the cam portion 84 of the slot 83 This mitted to the plate support H for the movable pawl 58 by means of the other roller 8|a.which engages the vertical slot 82 in that member. The proportions of the slots are so arranged that each movement of the trigger 28 from the outer extreme position of .FigurelO to the first intermediate position of Figure. 11 will rotate the magazine 58 the correct distance to bring each of the radial slots 88 therein into successivealignment with the axis of the barrel. In this position each tack carried by the magazine 58 is brought into alignment with a T-shaped opening 85 which extends through the plate 84 into the barrel 3|. The operating pawl 68 is resiliently biased into engagement with the magazine teeth 83 in order that the pawl support plate 'Il may retract from the indexing position to the tion without interference between the pawl and the magazine teeth.

for operating the indexing mechanism, an-outer plate section 81 provided with a slot opening 88 having a cam surface 88 for actuating the feed or injector mechanism for the tacks. This mechanism, as is illustrated particularly in Figures 9, 14, and 15, includes a thin, generally rectangularly shaped injector member 8| of nonmagnetic material having an elongated end section 82 adapted to engage a guide slot 88 formed in the central element 28 of the grip frame. Normally the injector member 8| is in the position. shown in Figure 2 and, by virtue of the fact that the injector cam 88 is spaced some distance in the rear of the indexing cam 14, the injector member 8| remains in this position'during the indexing operation.

' At the conclusion of the indexing operation, however,.that is, at the point illustrated in Figure 11, further inward movement of the trigger 28 will be eifective to raise the injector member 8|, this movement of the injector member 8| being accomplished by a link 85'provided with a roller 88- for engaging the slot 88 and the cam- 88. The link member 85 is pivotally supported at one of its ends upon an upstanding support normal position following each indexing opera- 81 ailixed to the grip frame, and at the other of its ends is provided with aslot 88 adapted to engage'a pin 88 integrally attached to the injector member 8|. Thus, movement of the trigger 28 from the first intermediate position illustrated in Figure 11 to the second intermediate position illustrated in Figure 14 serves to raise the injector member 8| through one of the radial slots 88 of the magazine 58 and to move the nail or tack located in that slot through the T-shape'd passageway 85 into the barrel 3| of the hammer mechanism. I'he head section 48 of the hammer 41, as mentioned previously, is magnetized to a very considerable degree, and by virtue of the resultant attraction, the head of the tack is drawn into position against the head of the hammer automatically as an incident to the upward movement of the injector '8|. The, roportions.

of the cam 88 and the cooperating elements are such that at the uppermost limit of its movement the injector 8| does not project into the barrel 8| of the hammer mechanism.

' In view of the fact that the slots as and as in both sections of the trigger 29 are'horizontal at their inner ends, the indexing mechanism and the injector mechanism are-held in, a static position when the trigger 28 is moved from the second intermediate position illustrated in Figure 14 to the extreme inner position illustrated in Figure 15. This movement of the trigger 28-from the second intermediate position to the extreme inner position is availed of for actuating the valve -mechanism and permits repeated blows tobe sive feeding of the next tack. The trigger 28 is guided and the limits of its movement are established by pins I80 projecting inwardly from the 5 01' the trigger 29 whereby movemcntjofthe trigger. serves to rotate the bell crank valve operating-- lever IL The inner end of the valve operatinglever IOI is also provided with a roller member I09 which is adapted to abut against the head 40 of the valve operating member as illustrated in Figures 14 .and 15. It will be seen, therefore,

that when the trigger ismoved from the second intermediate position shown in Figure 14 to the extreme inner position shown in Figure 15', the slide valve 35 will be operated to admit pressure air to the outer end of the piston chamber and move the hammer 41 through the barrel 3| carrying the tack with it. Ordinarily, one blow will sufllce to drive a tack but, if necessary, as many repeated blows as may be desired can be given by releasing the trigger 29 to the position shown in Figure 14 and returning it to the fully retracted position, i. e. the position shown in Figure 15. 'When the trigger has been completely released, i. e*. returned tothe position of Figure 2, the device is in operative position-to drive another tack. v

The operational cycle of the device of my invention may be briefly summarized as follows:

- 30. Initial-movement of the trigger 29 from the normal. or outer extreme position illustrated in Figures 2 and 10 to the first intermediate position illustrated in Figure ll will actuate the in dexing mechanism to cause rotation of the maga-' zine 59 in a clockwise direction-(Figure 5) to align the next succeeding magazine slot 60 with the axis of the barrel 3|, i. e. to a position where the tack located in that slot aligns with the T-shaped opening 85 extending into the barrel. 40 Further movement of the trigger 29 from the first intermediate'position of Figure 11 to the second intermediate position of Figure 14 effects the actuation of the injector mechanism, and the injector 9| pushes the adjacent overlying tack 45 into the barrel chamber; the magnetic attraction of the head portion 49 of the hammer 4T acts upon the tack as it is moved out of the magazine slot and draws the tack into driving position at the head end of the hammer. Additional move- 50 ment of the trigger from the second intermediate position of Figure 14 to the inner extreme position of Figure 15 actuates the valve mechanism 35 and causes the hammer 41 to move from the retracted position of Figure 2 to theextended or I driving position of Figure 15 with a blow action,

thereby driving the tack held onits head portion 48. l 1

Complete release of the trigger 29 will restore the device to its normal condition, and to drive a subsequent tack it is necessary merely to pull the trigger from the outer extreme positionof' Figure 2 to the inner extreme position of Figure 15 which will effect a repetition of the sequence of operations described above.

Ordinarily one blow of the hammer 41 will drive the tack, but if the initial blow is insufficient, as

many additional blows as may be required may be.

struck in sequence without interfering-with the normal successive feeding of tacks by simply releasing the trigger 29 so as to permit it tomove from the inner extreme position (Figure 15): to

the second intermediate position (Figure 14) whereupon the hammer will be retracted without 75 movement of the indexing or injector mechanisms, and then restoring the trigger 29 to the inner extreme position of Figure 15, which will effect reoperation or the hammer. The device inherently includes positive indicating means whereby the operator may readily determine just how far the trigger 29 should be released it further hammer blows are desired in the driving of any particular tack. There are three ,of these indicating-means: One, the operator can readily feel the retraction of the hammer 41 and stop forward movement or the trigger at the point where retraction occurs, 1. e. at the second intermediate position; two, the operator can stop the forward release or the trigger 29 when he hears the sound of. the escape of the air used on the driving stroke; and three, which is the most important 01 all, the substantial biasing force of the spring 39 is released from the trigger 29 at the second intermediate position, the point where the hammer is retracted without movementof the indexing or injector mechanism, thereby making it necessary for the operator to consciously decrease the retarding force of his finger at thispoint in order to fully release the trigger- The eifect is to cause therelease or the trigger 29 to take place in definite steps.

It .will be apparent from the foregoing that the device of my invention constitutes an easily operated means for automatically driving any number of tacks, nails or the like, in succession. The

position in which the tacks or nails are driven may be very accurately controlled-by merely plac-- ing the barrelof the deviceover the pointwhere it is desired. to drive the tack. The device is extremely flexible in operation in that it permits re-insertion of magazines simply by pressing the release latch 30a to open the magazine chamber .28, and it permits repeated blows to be given to. any, particular tack or nail without interfering with the normal automatic feed of the device.

The improved operation of my mechanism is' believed to be in part due to the novel magnetic magazine which I have evolved, and the structural features of the indexing and injecting mechanism. It will be evident that the principles which I have disclosed in the foregoing may be embodiedinto mechanisms of various types, and it is my desire, therefore, that the accompanying claims shall be accorded the broadest possible construction consistent with the state of the art. I claim the following as my invention: 1. In apparatus of the class described, hammer Y means operable to deliver a driving blow to atack,

a magazinetor supporting a plurality of tacks, a

trigger member movable between two extreme positions, means actuable in response to movement of said trigger from one of said extreme positions to an intermediate position to index said magazine, means operable in response to further 0 V movement of said trigger from said first intermediate position to a second intermediate position to move individual tacks successively from said magazine into operative relation with said ham-- 'mer'means, and means actuable in response'to movement of said trigger member from said sec- 0nd intermediate position to the other extreme position to operate said hammer means to deliver a driving blow to the tack which is in operative relation therewith, said last mentioned means being actuable to cause repeated'blows to'be delivered to said tack in response to repeated move-- ment of said trigger member between said second intermediate position and said other extremeposition.

2. A magazine for use in conjunction with apparatus of the class described, comprising a generally circular plate member of magnetic material 3. A magazine for apparatus 01 the class described, comprising a generally circular plate member of magnetic material provided with a plurality of radial slotsfor receiving th ishanks of tacks or nails, said slots defining spokes which are magnetized in such manner that the outer ends of adjacent spokes are of opposite polarity. 4. In apparatus of the class described, tack driving means which is operable to deliver a driving blow to atack and which includes a hammer member having a magnetized head, a magnetized magazine which comprises a generally circular plate of magnetic material provided with a plurality of radial slots for receiving the shanks of a plurality of tacks releasably held therein by magnetic attraction, an operating member, tack feeding means actuable in response to movement of said operating member to forcibly propel one of said tacks out of said magazine and toward said magnetized head, said magazine being so positioned relative to said magnetized head that the magnetic forces exerted by the magnetic fields of both said magazine and said head act simultaneously on each tack so propelled whereby each such tack is guided along a predetermined path into driving position on said magnetized head, and means also operable in response to movement of said actuating member to operate said hammer movement of said trigger member from said firstintermediate position to a second intermediate [position to move individual tacks successively from said magazine into operative relation with said hammer means, means actuable in response to movement of said trigger member from said second intermediate position to said innerextreme position to operate said hammer means to deliver a driving blow,'said hammer operating means after delivering a driving blow as aforesaid being restored to its normal operative condition by movement of said trlgger'member from said inner extreme position to said second intermediate position, and biasing means opposing movement of said trigger member from said outer 6O extreme position to said intermediate positions and to said inner extreme position, said biasing.

means being automatically actuable to materially decrease the biasing action on said trigger mem ber when said trigger member is moved from said inner extreme position to said second intermediate position whereby said trigger member may be moved between said second intermediate position and said inner extreme position to cause repeated blows to be delivered by said hammer means to said tack without operation of said indexing means or said tack injecting means.

6. In apparatus of the class described, hammer means operable to deliver a driving blow to--a tack, a tack magazine, a trigger member movable tack feeding means actuable in response to movement of said trigger member from the outer ex- .treme position to. an intermediate position to move individual tacks successively from said magazine into operative relation with said hammer means, means actuable in response to movement of said trigger member from said intermediate position to said inner extreme position to operate said hammer means to deliver'a driving blow, said hammer operating means after delivering a driving blow as aforesaid being restored to its normal operative condition by movement of said trigger member from said inner extreme position to said intermediate position, biasing means opposing movement of said'trigger member from said outer extreme position to said intermediate position and to said inner extreme position, said biasing means being automatically actuable to materially decrease the biasing action on said trigger member when said trigger member is moved from said; inner extreme position to said intermediate position, whereby said trigger member may be moved between said intermediate position and said inner extreme position to cause repeated blows to be delivered by said hammer means without operation of said tack feeding means.

7. In apparatus of the class described, tack driving means which is operable to deliver a driving blow to a tack and which includes a hammer member having a magetized head, a magnetized magazine for releasably holding a plurality of tacks in predetermined feeding positionby virtue of magnetic attraction, an operating member, tack feeding means actuable in response to movement of said operating member toforcibly propel one of said tacks out of said magazine and toward said magnetized head, said magazine being so positioned relative to said magnetized head that the magnetic forces exertedby the magnetic fields of said magazine and said head act simultaneously on each tack s'o propelled whereby each such tack is guided along apredetermined path into driving position on said magnetized head, and means also operable in response to movement of said operating member to operate said hammer to deliver a driving blow.

8. In apparatus of the class described, tack driving means which is operable to deliver a driving blow to a tack and which includes a hammer member having a magnetized head, a magnetized magazine which releasably holds a plurality of tacks by magnetic attraction, an operating member movable between an outer and an inner extreme position, tack feeding means actuable in response to movement of said operating member from said outer extreme position to an intermediate position to forcibly propel one of said tacks out of saidmagazine and toward said, magnetized head, said magazine being so positioned relative to said magnetized head that the magnetic forces exerted by the magnetic fields of both said magazine and said head act simultaneously on each tack so propelled, whereby each such tack is guidv treme positionto operate said tack driving means to deliver a driving blow, said tack driving means after delivering a driving blow as aforesaid being restored to its normal operative condition by movement of said operating member from said inner extreme position to said intermediate position, biasing means opposing movement of said trigger member from said outer extreme position to said intermediate position and to the inner extreme position, said biasingmeans being automatically actuable to materially decrease the biasing action on said operating member when that member is moved from said inner extreme position to said intermediate position whereby said operating member can be moved between said intermediate position and said inner extreme position to cause repeated blows to be delivered by said tack driving means without operation of said tack feeding means. v

9. In apparatus of the class described, hammer means operable'to deliver a driving blow to a tack, a magazine for supporting a plurality of tacks, a trigger or operating member movable between an outer and an inner extreme position, tack feeding means actuable in response to movementof said trigger member from the outer extreme position to an intermediate position to move, individual.

ative relation with said hammer means, and means actuable in response to movement of said trigger member from said intermediate position to said inner extreme position to operate said hammer means to deliver a driving blow, said hammer operating means after delivering a driving blow as aforesaid being restored to its normal operative position by movement of sald'trigge'r member from said inner extreme position to said intermediate position, whereby repeated blows may be delivered to each tack by repeating the movement of said trigger member between said 

